Particles are added to enhance and modify the properties of many different types of compositions and products. Examples include ultra-violet (UV) light absorbing particles, pigments, colorants, fillers, matting agents, optical diffusing particles, abrasion resistant particles, viscosity modifiers, magnetic particles and reflective particles. Especially in the case of nanoparticles, a very small weight percent (wt %) of particles added to the composition or product can dramatically affect properties. In order to be effective at such low weight percents, the particles must remain dispersed and chemically stable, during both the production and use of the composition or product. These problems are exacerbated as the dimensions of the particles are reduced because of the increase in total surface area, on a weight basis.
Chemical instability can result from reaction of the particles with other reagents, as well as with agents present in the environment, during any of the phases of the composition or product, such as manufacture, storage and use. Chemical instability may be exacerbated by environmental factors, such as exposure to visible and UV light, or exposure to elevated temperatures. Particle aggregation or poor dispersability is often the result of incompatibility of the particle surface with fluid components, especially incompatible hydrophobic/hydrophilic and electrostatic interactions with solvents or other particulate additives. Particle aggregation or poor dispersability may also be exacerbated by environmental factors, such as exposure to elevated temperatures, or long storage times. For large scale transport and ease of handling, it is often desirable to prepare liquid dispersions with high weight loading of the nanoparticles.
Particles comprising oxides are particularly suitable as additives, especially particles containing zinc oxides, titanium oxides, silicon oxides, aluminum oxides, iron oxides and/or rare-earth metal oxides. These oxides are thermodynamically stable, are typically unable to react with environmentally ubiquitous oxygen, and tend to be less reactive with water than many other oxides and non-oxide materials. These oxide materials have been used as pigments and abrasives for centuries. Nanoparticles consisting of certain metal oxides, most notably titanium oxides, are particularly interesting for use in coating compositions, because they are usually colorless and transparent to visible light, and provide protection against exposure to UV light; however they tend to have poor photostability, caused by the photocatalytic behavior of these oxides. In cosmetic preparations, poor photostability often manifests as a color change and is not acceptable for commercial topical skin products. Poor photostability also interferes with use in paints or other product coatings, resulting in reactivity and “chalking out”.
In order to improve dispersability in non-aqueous fluids, particles have been coated or surface treated with hydrophobic reagents. Coatings and surface treatments have also been used to enhance chemical stability, including the photostability of titanium and other oxides.
T-Cote 031 is a microfine titania (titanium oxide) with a mean particle size of less than 200 nm which has been treated with dimethicone (poly(dimethylsiloxane)) on the particle surface. The hydrophobic dimethicone surface treatment provides compatibility with non-aqueous oils that serve as liquid carriers in a variety of products. While this nanoparticle material is used to produce dispersions at high weight loading, a major deficiency in photostability prevents its use in dispersions intended for commercial use. The performance of T-Cote 031 indicates that a simple dimethicone treatment is not sufficient to enhance photostability, as the photostability of this material is nearly indistinguishable from uncoated microfine titania.
Aeroxide T805 is a fumed titanium dioxide powder which has been treated to form octyl silane (H(CH2)8Si(O)3) moieties on the particle surface. Presumably, the octyl silane coating is applied by reacting the particle surface with a trifunctional alkoxy octylsilane such as triethoxy octylsilane. While this treatment does not render the titania surface completely inert, it is sufficiently chemically stable for some commercial applications. Aeroxide T805 is sufficiently photostabile for use as an additive in a cosmetic preparation, and is currently used in several topical human sunscreens. High solids dispersions are highly desirable in sunscreen formulations since they enable high SPF (Sun Protection Factor) values to be achieved while introducing a minimal amount of carrier fluid. However, difficulty is encountered when high solids dispersions are formulated; typically a paste is formed. These high solids dispersions are commercially available, but due to high viscosity, they are difficult to mix with other reagents and are prone to waste since it is difficult to remove all of the material from the storage container.